In recent years, in the field of optical communication or optical measurement, waveguide-type optical elements such as optical modulators having an optical waveguide formed on a substrate having an electro-optic effect have been frequently used. Waveguide-type optical elements, generally, include the optical waveguide and a control electrode for controlling an optical wave that is transmitted through the optical waveguide.
As the above-described waveguide-type optical elements, for example, Mach-Zehnder-type optical modulators in which lithium niobate (LiNbO3) (also referred to as “LN”), which is a ferroelectric crystal, is used for the substrate are widely used. Mach-Zehnder-type optical elements includes a Mach-Zehnder-type optical waveguide comprising an incident waveguide for introducing light from the outside, a branch portion for transmitting the light introduced using the incident waveguide to two separate paths, two parallel waveguides for transmitting the respective branched light rays at the rear end of the branch portion, and an emission waveguide for coupling the light rays that are transmitted through the two parallel waveguides and outputting the light rays to the outside.
In addition, Mach-Zehnder-type optical modulators include a control electrode for changing and controlling the phase of an optical wave that is transmitted through the parallel waveguides using an electro-optic effect when a voltage is applied. Generally, the control electrode is constituted as a coplanar waveguide (CPW)-type electrode having a central electrode formed in the upper portion of the parallel waveguides or near the parallel waveguides in the length direction of the parallel waveguide and a ground electrode disposed away from the central electrode.
Particularly, in the design of broadband (microwave band) Mach-Zehnder-type optical modulators for controlling an optical wave that is transmitted through the parallel waveguides at a higher frequency, it is necessary to balance the following (1) to (3).
(1) Velocity matching between the transmission velocity of light that is transmitted through the parallel waveguides and the transmission velocity of light that is transmitted through the central electrode (hereinafter, simply referred to as “velocity matching”)
(2) Matching of the input impedance of the central electrode to the output impedance of the drive circuit (hereinafter, referred to as impedance matching)
(3) overlap of an optical wave and a microwave (modulation efficiency)
That is, in Mach-Zehnder-type optical modulators, as an essential condition for carrying out the velocity matching and the impedance matching, it is necessary to decrease the driving voltage as much as possible. Furthermore, in order to broaden the band, it also becomes necessary to reduce the loss of high-frequency signals (microwave loss) that are transmitted through electrodes. However, the decrease in the driving voltage and the broadening of the band has a conflicting relationship, and it is difficult to broaden the band without increasing the driving voltage.
In the related art, processing the surface of a substrate into a ridge shape and broadening the band while suppressing an increase in the driving voltage for achieving both the velocity matching and the impedance matching is known (Non Patent Literature No. 1).
In addition, in order to further broaden the band, a two-level constitution (that is, a constitution in which the thicknesses of these electrodes are changed to have two levels) is provided to the ground electrode and/or the central electrode, and thus the thicknesses of the electrodes at which the velocity matching is satisfied are increased without changing the impedance, thereby reducing the conductor loss of a microwave (Patent Literature No. 1).
In the above-described waveguide-type optical element of the related art, when the velocity matching is achieved using the substrate thickness, the electrode thicknesses, and the like, and the microwave loss is reduced, broadband action becomes possible to a certain extent. However, in actual cases, the upper limit of the thicknesses of electrodes that can be formed is approximately 50 μm, and there is a limitation in broadening the band by increasing the thicknesses of electrodes. That is, in the formation of electrodes, it is necessary to increase the thickness of a resist that is used for the patterning of the electrodes in accordance with the thicknesses of the electrodes to be formed. However, as the thickness of the resist increases, stress in the resist increases, the attaching force between the resist and the substrate decreases, and a possibility of the resist being peeled off in the middle of the formation of the electrode increases. In addition, the time taken to form the electrodes also increases in accordance with the thickness, and productivity degrades. Here, in a method for forming the electrodes, semi-additive in which a photoresist and electroplating are used is used as described in Non Patent Literature Nos. 1 and 2.